Degradation of Polymer Additives during Long-Term Zn Electrowinning

The time-dependence of molecular weight of gelatin, the cathode potential and the morphology of deposited Zn were investigated during long-term electrolysis of Zn to evaluate the degradation of gelatin and polyethylene glycol (PEG) added to the electrolyte for Zn electrowinning. Gelatin and PEG acted as a polarizer for Zn deposition, shifting the cathode potential to less noble direction. The polarization caused by the additives gradually decreased with increasing the electrolysis duration. The effective duration of additives significantly depended on the molecular weight and was longer in PEG-containing solution than in gelatin-containing solution. Gel permeation chromatography revealed that the molecular weight of gelatin in electrolyte decreased with increasing the electrolysis duration. The decrease in molecular weight of gelatin by electrolysis brought about the shift of the cathode potential to noble direction, showing that the molecular weight of degraded gelatin was related with the deposition potential of Zn. Even in the case without electrolysis, the molecular weight and polarization effect of gelatin decreased with increasing the lapse of time. On the other hand, the morphology and the crystal orientation of Zn obtained by long-term electrolysis corresponded to the change of the cathode potential with time regardless of the kinds of additives. With an increase in the cathodic polarization, the deposits showed the smaller grains with preferred orientation of (10-10). The morphology and the crystal orientation of deposited Zn became close to that in the additive-free solution when the polarization was decreased as a result of the degradation of additives.